UNITED NATIONS - A new round of diplomatic outreach with Iran will be among President Obama's top second-term priorities, he told the United Nations today, though voicing cautious skepticism that any talks will lead to a dramatic change in relations.

"I have made it clear that even when America's core interests are not directly threatened, we stand ready to do our part to prevent mass atrocities and protect human rights," Obama told members of the U.N. General Assembly. "Yet we cannot and should not bear that burden alone."

In a sweeping 45-minute address, Obama warned the world community that Syria's use of chemical weapons and Iran's contested nuclear program threaten the security of all nations, and could undermine the credibility of the United Nations.

"We will not tolerate the development or use of weapons of mass destruction," Obama said. "Just as we consider the use of chemical weapons in Syria to be a threat to our own national security, we reject the development of nuclear weapons that could trigger a nuclear arms race in the region, and undermine the global non-proliferation regime."

The president called for a Security Council resolution on Syria that would impose "consequences" on President Bashar al-Assad if he fails to comply with a promise to surrender his chemical weapons stockpiles within the year. Failure to do so, Obama said, would show the United Nations "incapable of enforcing the most basic of international laws."

Turning to Iran, Obama said a new round of diplomatic outreach with newly elected President Hassan Rouhani would be one of his top second-term priorities, while voicing cautious skepticism that it will lead to dramatic change.

"The United States and Iran have been isolated from one another since the Islamic Revolution of 1979. This mistrust has deep roots," Obama said.

"I don't believe this difficult history can be overcome overnight; the suspicion runs too deep. But I do believe that if we can resolve the issue of Iran's nuclear program, that can serve as a major step down a long road towards a different relationship, one based on mutual interests and mutual respect."

The Obama administration Monday said Secretary of State John Kerry would participate in multilateral negotiations over Iran's nuclear program, meeting later this week with counterparts from the European Union, the United Kingdom, France, Germany, Russia and China. The gathering will also include Iranian foreign minister Javad Zafir, marking the highest-level contact between the two nations on the nuclear issue in decades.

"To succeed, conciliatory words will have to be matched by actions that are transparent and verifiable," Obama said of friendly overtures from the Iranian regime.

While speculation continues to swirl about a possible Obama encounter with Rouhani on the sidelines of the U.N. meeting, U.S. officials say there are no plans for the leaders to meet. But they have not shut the door entirely.

Obama's fifth address to the annual meeting of world leaders and dignitaries sought to reframe the U.S. approach to the Arab Spring after a tumultuous year across the Middle East and North Africa.

In a shift, Obama said efforts to broker a peace deal between the Israelis and Palestinians would have added urgency in his second term, calling the festering dispute a "major source of instability" for too long.